Voltage tunable laminated dielectric materials for microwave applications

ABSTRACT

A tunable dielectric structure includes a first layer of dielectric material, a second layer of dielectric material positioned adjacent to the first layer of dielectric material, with the second layer of dielectric material having a dielectric constant that is less than the dielectric constant of the first layer of dielectric material, and electrodes for applying a controllable voltage across the first dielectric material, thereby controlling a dielectric constant of the first dielectric material, wherein at least one of the electrodes is positioned between the first and second layers of dielectric material. The dielectric materials can be formed in various shapes and assembled in various orientations with respect to each other. The tunable dielectric structure is used in various devices including coaxial cables, cavity antennas, microstrip lines, coplanar lines, and waveguides.

CROSS REFERENCE TO RELATED PATENT APPLICATION

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/419,047, filed Oct. 15, 1999, which claims thebenefit of U.S. Provisional Application No. 60/104,503, filed Oct. 16,1998.

BACKGROUND OF INVENTION

[0002] The present invention relates generally to electronic materialsfor microwave applications, and more particularly to such materialsincluding ferroelectric materials having a tunable dielectric constant.

[0003] Tunable ferroelectric materials are the materials whosepermittivity (more commonly called dielectric constant) can be varied byvarying the strength of an electric field to which the materials aresubjected or immersed. Even though these materials work in theirparaelectric phase above Curie temperature, they are conveniently called“ferroelectric” because they exhibit spontaneous polarization attemperatures below the Curie temperature. Typical tunable ferroelectricmaterials are barium-strontium titanate (BST) or BST composites.Examples of such materials can be found in U.S. Pat. Nos. 5,312,790,5,427,988, 5,486,491 and 5,643,429. These materials, especially BSTO—MgOcomposites, show low dielectric loss and high tunability. Tunability isdefined as the fractional change in the dielectric constant with appliedvoltage. These unique properties make these materials suitable formicrowave applications such as phase shifter, tunable filters, tunableresonators, and delay lines.

[0004] U.S. Pat. No. 5,830,591 discloses a multi-layer ferroelectriccomposite waveguide in which the effective dielectric constant of thewaveguide can be reduced while maintaining tunability. The waveguide isconstructed of high and low dielectric constant layers. The multi-layerwaveguide is comprised of bias plates that are perpendicular to thelaminate direction to maintain tunability in the structure. Thestructure disclosed in U.S. Pat. No. 5,830,591 is only suitable forwaveguide applications. Since high dielectric fields, for example about10 V/μm, are necessary to tune tunable material, especially in waveguideapplications, the distance between bias electrodes should be kept small.With the bias plate arrangement of U.S. Pat. No. 5,830,591, multiplelayers would be needed along the direction of that bias plates as wellas in the direction of the laminated dielectric material stack. Thismakes fabrication of such devices complex.

[0005] U.S. Pat. No. 5,729,239 discloses a device for scanning in ascanning plane that includes a periodic array of conductive platesdisposed along the scanning axis, adjacent plates being disposed abouthalf a wavelength apart. The device has a periodic array of slabsdisposed along the scanning axis, each slab comprising ferroelectricmaterial, being disposed between a pair of adjacent conductive plates ofthe periodic array of conductive plates, with adjacent slabs beingseparated by one of the conductive plates. Each of the slabs has areceiving face and a radiating face substantially parallel to eachother. Each of the slabs transmits an electromagnetic signal from thereceiving face to the radiating face. Input transmission means feed aninput electromagnetic signal to the periodic array of slabs in apropagation direction so that the input electromagnetic signal isincident on the receiving faces of each of the slabs and so that theelectrical component of the input electromagnetic signal received ateach receiving face has a component parallel to the scanning axis.Output transmission means transmit an output signal from the periodicarray of slabs responsive to the electromagnetic signal transmitted fromeach receiving face in the corresponding slab. The device also includesa plurality of means for selectively applying a voltage across each ofthe pairs of conductive plates disposed about a slab so as toselectively control the phase of the electromagnetic signal received ateach of the radiating faces having been transmitted from the receivingface in the corresponding slab.

[0006] U.S. Pat. No. 5,729,239 discloses the use of barium strontiumtitanate (BSTO), or composites thereof, or other ceramics as theferroelectric material. BSTO—MgO has also been proposed by others foruse as a tunable ferroelectric material. However, the materials in theBSTO—MgO system generally have dielectric constants of over 100. Thehigh dielectric constant is not suitable for some microwave applicationssuch as patch antennas, which lower the antennas' efficiencies. Highdielectric constant materials also cause low characteristic impedance(<10 Ω) in microstrip, coplanar, and other planar structure transmissionlines, which strongly limits the application of high dielectric constantmaterials. Low dielectric constant materials (for example, withdielectric constants less than 40) with low loss and high tunability aredesired for patch antennas and other microwave applications.

[0007] It would be desirable to construct a ferroelectric structurehaving a relatively low overall dielectric constant that takes advantageof the high tunability and low loss characteristics of materials such asBST composites, having high dielectric constants. It is further desiredto construct such structures for use in various microwave devices suchas microstrips, coplanar or other planar microwave transmission lines,coaxial cable, or waveguides.

SUMMARY OF THE INVENTION

[0008] This invention provides a tunable dielectric structure includinga first layer of dielectric material, and a second layer of dielectricmaterial positioned adjacent to the first layer of dielectric material,with the second layer of dielectric material having a dielectricconstant that is less than the dielectric constant of the first layer ofdielectric material. The structure further includes electrodes forapplying a controllable voltage across the first dielectric material,thereby controlling a dielectric constant of the first dielectricmaterial, wherein at least one of the electrodes is positioned betweenthe first and second layers of dielectric material.

[0009] The dielectric materials can be formed in various shapes andassembled in various orientations with respect to each other. Laminatedstructures of such dielectric materials can serve as substrates formicrostrips, coplanar or other planar microwave transmission lines, aswell as dielectric media for coaxial cable or waveguides.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A full understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

[0011]FIG. 1 is isometric view of a dielectric structure of thelaminated material constructed in accordance with a preferred embodimentof the invention;

[0012]FIG. 2 is schematic representation of a laminated structure inaccordance with the invention;

[0013]FIG. 3 is a schematic of the equivalent electric circuit of alaminated structure in accordance with the invention;

[0014]FIG. 4 is an end view of a coaxial line that includes a dielectricstructure of the laminated material constructed in accordance with theinvention;

[0015]FIG. 5 is a cross sectional view of the structure of FIG. 4, takenalong line 5-5;

[0016]FIG. 6 is an end view of another alternative embodiment of theinvention for antenna applications;

[0017]FIG. 7 is a cross sectional view of the structure of FIG. 6, takenalong line 7-7;

[0018]FIG. 8 is isometric view of a microstrip line that includes adielectric structure of the laminated material constructed in accordancewith the invention;

[0019]FIG. 9 is isometric view of a coplanar line that includes adielectric structure of the laminated material constructed in accordancewith the invention; and

[0020]FIG. 10 is an isometric view of a waveguide that includes adielectric structure of laminated material constructed in accordancewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Referring to the drawings, FIG. 1 is an isometric view of anelectronic device having a dielectric structure of laminated materialsconstructed in accordance with a preferred embodiment of the invention.The tunable dielectric device 10 comprises a multilayered structure ofdielectric materials 12, 14, 16, 18 including two or more materialshaving different dielectric constants, and being laminated together totailor the overall dielectric constant and tunability. One or morematerials (e.g. 12 and 16) in the laminated structure are tunabledielectric materials usually with a high dielectric constant, lowlosses, and high tunability. For the purposes of this description, ahigh dielectric constant is greater than about 100, low loss materialshave loss tangents (tan δ) of less than about 0.01, and tunability ofgreater than about 15% at 2 V/μm. The high dielectric constant, low lossand high tunability materials may be Ba_(1-x)Sr_(x)TiO₃ (BSTO), where xcan vary between zero and one, and composites thereof that exhibit suchlow losses and high tunability. Examples of such composites include, butare not limited to: BSTO—MgO, BSTO—MgAl₂O₄, BSTO—CaTiO₃, BSTO—MgTiO₃,BSTO—MgSrZrTiO₆, or any combination thereof. The other materials (e.g.14 and 18) in the laminated structure may be tunable or non-tunabledielectric materials such as Ba_(1-x)Sr_(x)TiO₃-based materials,alumina, Mica, and air. Since air is the lowest dielectric material andthe lowest loss material, it is particularly useful for certainapplications. If air is used as the non-tunable dielectric in thestructures of this invention, the tunable materials would be mountedwith air gaps between tunable layers. The resultant overall dielectricconstant, tunability and other properties of the laminated material isdependent on the relative properties and thickness of each of thelayered materials. Therefore, these properties can be tailored byvarying the number of layers of dielectric materials with certaindielectric constant characteristics, and varying the thickness of thelayers.

[0022] The laminate material of the present invention appears as auniform material to a radio frequency signal that is applied to thestructure. While the structures of the invention are not limited to anyparticular dimensions, the thickness of the layers should be such thatthis apparent uniformity is achieved. In the preferred embodiments, thatthickness of the dielectric layers is smaller than one-tenth of thewavelength of the radio frequency signal to be used with the device.

[0023] In the embodiment of FIG. 1, each of the dielectric materials isin the form of a rectangular slab. Each slab has an input end 20 forreceiving a radio frequency signal (RF_(IN)) and an output end 22 fordelivering the signal (RF_(OUT)). In general, the laminated structure ofFIG. 1 can serve as a tunable dielectric media for microwavetransmission. The means for inputting and outputting a radio frequencysignal from the structure will depend upon the application in which itis used. Electrodes 24, 26, 28, 30 and 32, in the form of sheets ofconductive material are located at each end of the stack and betweeneach of the tunable dielectric materials. The electrodes are positionedadjacent to opposite faces of at least each slab that is comprised oftunable ferroelectric material. With this structure, at least some ofthe electrodes are positioned within the laminate stack and lie inplanes parallel to the direction of propagation of the RF signal andparallel to opposite faces of the slabs of tunable material. For thosedielectric material slabs that have a voltage controlled dielectricconstant, a controllable DC voltage source 34 is electrically connectedto the electrodes on opposite sides of the slab. In FIG. 1, only onecontrollable DC voltage source is shown, but it must be understood thatadditional voltage sources may be used to control the dielectricconstant of the several slabs, or the same DC voltage source may beconnected to multiple slabs of dielectric material. In the preferredembodiments, layers of the same tunable dielectric material would besubject to the same bias voltage. In addition, the polarity of theapplied voltage can be changed without affecting performance of thedevice. A coordinate system is illustrated in FIG. 1 such that the slabslie in planes parallel to the y-z plane, and are stacked in the xdirection. The radio frequency signal propagates in the y directionthrough the device.

[0024]FIG. 2 is a schematic representation of a laminated structure inaccordance with the invention. In the embodiment depicted in FIG. 2, aplurality of slabs 36, 38, 40 and 42 of dielectric material are shown tohave dielectric constants of ε1, ε2, ε3, through εn, and thickness t1,t2, t3, through tn, respectively. FIG. 2 shows a structure that includestwo assemblies 44 and 46, each having the same arrangement of dielectricmaterials. A plurality of electrodes, for example 48, 50 and 52 arepositioned between the dielectric slabs and are connected to one or morecontrollable DC voltage sources. In FIG. 2, one controllable voltagesource 54 is shown for clarity. However, as discussed above multiplesources, and/or multiple connections to a single source may be used inoperational devices. This figure illustrates that a complete device canbe comprised of multiple subassemblies, each having the same or asimilar arrangement of dielectric materials. Coordinates x, y and z inFIG. 2 correspond to coordinates x, y and z in FIG. 1.

[0025]FIG. 3 is a schematic of the equivalent electric circuit of alaminated structure in accordance with the invention. In FIG. 3, atleast selected ones of the various values of capacitance C₁, C₂, C₃,through C_(n), can be changed by varying the control voltages applied tothe dielectric slabs that contain tunable ferroelectric material. Theoverall capacitance of the laminated structure is the sum of thecapacitance of the individual slabs.

[0026]FIG. 4 is an end view of an alternative embodiment of theinvention for tunable coaxial cable applications in which the dielectricmaterial is arranged in concentric cylinders 56, 58, 60, and 62. Hereagain, some of the layers of dielectric material can be tunable materialhaving relatively high dielectric constants, low losses and hightunability, while the other layers can be tunable or non-tunablematerial. Concentric cylindrical electrodes 64, 66, 68 and 70 arepositioned between the dielectric materials so that a bias voltage canbe applied to the control the dielectric constants of the dielectriccylinders that contain tunable ferroelectric material. A metallic centerconductor 72, and a cylindrical metallic ground 74 are provided to carrythe RF signal through the cable.

[0027]FIG. 5 is a cross sectional view of the structure of FIG. 4, takenalong line 5-5. One of the controllable DC voltage sources is shown tobe connected to electrodes 68 and 70. Additional controllable voltagesources (not shown) would be used for applying bias voltages to otherelectrodes that lie adjacent to the internal and external surfaces ofthe tunable layers. The direction of propagation of a radio frequencysignal through the structure is illustrated by arrow 76. The cylindricalelectrodes are positioned around an axis that lies parallel to thedirection of propagation of the radio frequency signal through thedevice. In FIG. 5, items 56, 58, 60, 62, 64, 72 and 74 identify thatsame structures as identified by those item numbers in FIG. 4.

[0028]FIG. 6 is an end view of another embodiment of the invention inthe form of a tunable cavity antenna that includes a plurality ofrectangular slabs of dielectric material, a representative sample ofwhich are numbered as items 82, 84, 86 and 88. Each of the slabs has apair of electrodes, as illustrated by items 90 and 92, on opposite sidesthereof for the application of a bias voltage. As shown in the figure,the slabs are arranged such that certain slabs lie in planes that areperpendicular to the planes occupied by certain other slabs. Thisinvention provides a tunable cavity for a cavity antenna by placing alaminated, tunable material, with a specific dielectric constant, intothe cavity. The open spaces in the cavity of FIG. 6 can be filled withair or a non-tunable dielectric material.

[0029]FIG. 7 is a cross sectional view of the structure of FIG. 6, takenalong line 7-7. In FIG. 7, items 82, 84, 86, 88, 90 and 92 identify thesame structures as identified by those item numbers in FIG. 6. Acontrollable voltage source 94 is shown to supply a controllable biasvoltage to electrodes 90 and 92, thereby controlling the dielectricconstant of the dielectric material 82. While only one controllablevoltage source is shown, it will be appreciated by those skilled in theart that additional controllable voltage sources, or alternativeconnects to a single source, would be used to practice the invention.Arrow 96 illustrates the direction of propagation of a radio frequencysignal through the device. In the structure of FIGS. 6 and 7, selectedones of the dielectric slabs can contain material having a relativelyhigh dielectric constant, low losses, and high tunability. The otherslabs of dielectric material can be tunable or non-tunable materials.

[0030]FIG. 8 is an isometric view of a microstrip line that includes adielectric structure of the laminated material constructed in accordancewith the invention. In this embodiment, a laminated structure 10′similar to that of FIG. 1 is constructed of a plurality of slabs ofdielectric material, as illustrated by items 98, 100, 102 and 104. Hereagain, electrodes are positioned on opposite sides of the slabs, asillustrated by electrodes 106, 108, 110, and 112. The laminatedstructure 10′ is mounted on a ground plane 114 such that the slabs arepositioned generally perpendicular to the ground plane. A microstrip 116is mounted on a side of the laminated structure opposite the groundplane.

[0031]FIG. 9 is an isometric view of a coplanar line that includes adielectric structure of the laminated material constructed in accordancewith the invention. In this embodiment, a laminated structure 10″similar to that of FIG. 1 is constructed of a plurality of slabs ofdielectric material, as illustrated by items 118, 120, 122, and 124.Here again, electrodes are positioned on opposite sides of the slabs, asillustrated by electrodes 126, 128, 130, and 132. A center strip 134 andtwo ground planes 136 and 138 are mounted on one side of the laminatedstructure. The ground planes are positioned on opposite sides of thecenter strip. The RF signal is transmitted through the center strip andthe adjacent ground planes.

[0032]FIG. 10 is an isometric view of a waveguide that includes adielectric structure of laminated materials constructed in accordancewith the invention. In this embodiment, a laminated structure 10′″similar to that of FIG. 1 is constructed of a plurality of slabs ofdielectric material, as illustrated by 140, 142, 144, and 146. Hereagain, the electrodes are positioned on opposite sides of the tunableslabs, as illustrated by electrodes 148, 150, 152 and 154. The laminatedmaterial is filled into the waveguide 156 in a manner similar to thatused for known dielectric loaded waveguides. The RF signal is then inputand output in accordance with known techniques.

[0033] The laminated dielectric material structure of the presentinvention can provide certain overall dielectric constant(s) andtunability by laminating high dielectric constant, high tunabilitymaterial(s) with low dielectric constant tunable or non-tunablematerial(s) without substantial lowering of their tunability, ordegradation of dielectric loss. For the purposes of this invention, highdielectric materials have a dielectric constant greater than about 100,and low dielectric materials have a dielectric constant lower than about30.

[0034]FIG. 1 illustrates the concept of the present invention, whichconsists of n(n≧2) layers of different materials with dielectricconstants ε_(n), and thickness t_(n). Since the equivalent circuit isthat of parallel capacitors, the resultant dielectric constant ε_(e) ofthe laminated material is expressed as following: $\begin{matrix}{{ɛ_{e}^{0} = {\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{0}}}};\quad \left( {T = {\sum\limits_{i = 1}^{n}\quad t_{i}}} \right)} & (1)\end{matrix}$

[0035] here ε_(e) ⁰ is the resultant dielectric constant of thelaminated materials at no dc bias, t_(i) is the thickness of the ithlayer, T is the total thickness of the laminated materials, ε_(i) ⁰ isthe dielectric constant of the ith layer at no dc bias.

[0036] Under dc bias conditions: $\begin{matrix}{{ɛ_{e}^{v} = {\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{v}}}};} & (2)\end{matrix}$

[0037] where ε_(e) ^(ν) is the resultant dielectric constant of thelaminated materials at a dc bias of field E, and ε_(i) ^(ν) is thedielectric constant of the ith layer at dc bias of field E. Then:

ε_(i) ^(ν)=ε_(i) ⁰(1−b _(i)E);  (3)

[0038] where b_(i) is the tunability of ith material, which is definedas: $\begin{matrix}{{b_{i} = {- \frac{\frac{ɛ_{i}^{v}}{E}}{ɛ_{i}^{0}}}};} & (4)\end{matrix}$

[0039] Therefore, the tunability of the laminated material is:$\begin{matrix}{b_{e} = {{- \frac{\frac{ɛ_{e}^{v}}{E}}{ɛ_{e}^{0}}}\quad = \frac{\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{0}b_{i}}}{\sum\limits_{i = 1}^{n}\quad {\frac{t_{i}}{T}ɛ_{i}^{0}}}}} & (5)\end{matrix}$

[0040] In the case of all layers having the same tunability but withdifferent dielectric constants, equation (5) will be

b_(e)=b₁; I=1,2, . . . , n  (6)

[0041] Equation (6) indicates that the laminated material has the sametunability as the individual tunability of each layer.

[0042] Now a two-layer case is considered. Layer 1 is a tunable materialwith high dielectric constant. Layer 2 is a non-tunable material withlow dielectric constant. If t₁≡t₂ or t₁ is not much smaller than t₂, wecan get from equation (5) that

b_(e)=b₁;  (7)

[0043] Equation (7) indicates the laminated material tunability is thesame as the tunable layer.

[0044] This invention provides a multi-layered structure of highdielectric constant, low loss, and high tunability materials laminatedwith low dielectric materials, which may be tunable or non-tunable. Theinvention is not limited only to obtaining low dielectric constantmaterials. Any dielectric constant bounded by the dielectric constantsassociated with the individual layers can be achieved with this method.

[0045] The method of laminating different layers can be simplymechanical, co-firing, or thick film and/or thin film processing. Inthese methods, the properties of the individual layers should be thesame as, or close to the properties of the corresponding layers in thefinal laminated structure.

[0046] Accordingly, in the present invention, a laminated structurematerial can be realized by alternating two or more different dielectricconstant materials using either physical or chemical processing. Thedielectric constant can be tailored by choosing both proper materialsand layer thickness with little or no loss of tunability or degradationof dielectric loss.

[0047] An advantage of the present invention is that a certain overalldielectric constants can be easily tailored by laminating highdielectric constant material(s) with low dielectric constantmaterial(s). The resultant dielectric constant of the laminatedmaterial(s) can range from several to tens, even to hundreds ifnecessary, since the high dielectric material(s) may beBa_(1-x)Sr_(x)TiO₃ (BSTO), where x can vary between zero and one, andBSTO composites, with dielectric constants that vary from about 100 tothousands, and low dielectric constant material(s) such as air (ε=1),and/or other dielectric materials such as alumina (ε=9-10), Mica(ε=4.2), and Ba_(1-x)Sr_(x)TiO₃-based materials.

[0048] While the present invention has been disclosed in terms of itspresently preferred embodiments, it will be understood by those in theart that various modifications of the disclosed embodiments can be madewithout departing from the spirit and scope of this invention, which isdefined by the following claims.

What is claimed is:
 1. A waveguide comprising: a first layer ofdielectric material; a second layer of dielectric material positionedadjacent to said first layer of dielectric material, said second layerof dielectric material having a dielectric constant that is less thanthe dielectric constant of said first layer of dielectric material;first and second electrodes for applying a controllable voltage acrosssaid first dielectric material, thereby controlling a dielectricconstant of said first dielectric material, wherein at least one of saidelectrodes is positioned between said first and second layers ofdielectric material; a microstrip positioned adjacent to a first edge ofeach of said first and second layers; and first and second ground planespositioned on opposite sides of said microstrip.
 2. A waveguide asrecited in claim 1, further comprising: means for applying acontrollable voltage across said second dielectric material, therebycontrolling a dielectric constant of said second dielectric material. 3.A waveguide as recited in claim 1, further comprising: a plurality ofadditional layers of dielectric material positioned generally inparallel with said first and second layers of dielectric material, atleast selected ones of said additional layers of dielectric materialhaving a tunable dielectric constant.
 4. A waveguide as recited in claim1, wherein said first layer of dielectric material has a dielectricconstant greater than about 100 and a loss tangent less than about 0.01.5. A waveguide as recited in claim 1, wherein said second layer ofdielectric material comprises one of: a Ba_(1-x)Sr_(x)TiO₃ compositewhere x ranges from zero to one, alumina, mica, and air.
 6. A waveguideas recited in claim 1, wherein said first and second layers aregenerally rectangular slabs lying in planes that are oriented parallelto a direction of propagation of a radio frequency signal through thewaveguide.
 7. A waveguide as recited in claim 1, wherein said firstdielectric material comprises one of: BSTO, BSTO—MgO, BSTO—MgAl₂O₄,BSTO—CaTiO₃, BSTO—MgTiO₃, BSTO—MgSrZrTiO₆, or a combination thereof.